Contours of instantaneous values of normal velocity component in turbulent channel flow illustrating the effects of rotation on the structure of turbulence.
Ravikanth Avancha, Department of Mechanical Engineering, Iowa State University, Ames, Iowa
Isaac Giesen, Undergraduate Student Researcher
Joon Lee, Department of Mechanical Engineering, Iowa State University, Ames, Iowa
Ning Meng, Department of Mechanical Engineering, Iowa State University, Ames, Iowa
Richard Pletcher, Department of Mechanical Engineering, Iowa State University, Ames, Iowa
Todd Simons, Graduate Student Researcher
Xiaofeng Xu, Department of Mechanical Engineering, Iowa State University, Ames, Iowa
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"Large Eddy Simulation of a Turbulent Channel Flow with a Rib-Roughened Wall," N. Meng, R.H. Pletcher, and T.A. Simons, Presented at the 37th AIAA Aerospace Sciences Meeting and Exhibit in Reno, NV, January 11-14, 1999. |
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"Large Eddy Simulation of Constant Heat Flux Turbulent Channel Flow with Property Variations Using a Dynamic Subgrid-Scale Model," N. Meng, L.D. Dailey, and R.H. Pletcher, in American Institute of Aeronautics and Astronautics paper 99-3356 (1999). |
This Principal Investigator has been working on experimental aspects of flows in gas turbine engines for several years. The motivation for the project is the fact that better computational methods are needed for predicting features of turbulent flows in certain critical components of gas turbine engines. The computational study can complement the experimental work underway at the University of Minnesota. With the present state of computer hardware, design codes are usually restricted to the use of relatively simple turbulence models. Unfortunately, flows in gas turbine engines are not simple, and an accurate turbulence model for film cooling flows and flows undergoing transition, in particular, have been very elusive. This research effort is applying large-eddy simulation (LES) to turbulent flows in configurations characteristic of those occurring in critical turbine components of aeropropulsion systems. Such a method is capable of achieving very realistic results because very little ad hoc modeling is employed to represent the effects of turbulence. The unsteady, three-dimensional motion of the large eddies is resolved from first principles and modeling is only used to account for the effects of eddies smaller than the computational grid itself. Such small eddies tend to be nearly isotropic and universal. The simulations should reveal aspects of the fundamental physics that are difficult or impossible to measure as well as providing information that should be useful in developing or refining turbulence models for design-oriented computer codes. Comparisons are being made with the extensive experimental results already obtained.
Recent work has led to the development of algorithms and methodology for direct and LES of turbulent flows with heat transfer using a compressible formulation of the Navier-Stokes equations with low Mach number preconditioning. With preconditioning, the numerical stiffness and slow convergence associated with traditional compressible formulations are avoided, and in fact, the convergence rate becomes virtually independent of Mach number in the low speed regime. Although some heat transfer results have appeared in the literature, available results are very limited, especially for separated flows, and usually do not take into account effects of variations in fluid properties.
Flows of interest include film and internal passage cooling flows in turbines and transition (by-pass and separation bubble) of the type occurring in low-pressure turbines operating at low Reynolds numbers. Over the past several decades, turbine inlet temperatures have increased significantly as higher efficiencies have been achieved. Hand-in-hand with this improvement has been the development of methods of cooling turbine blades to maintain structural integrity. Further improvements in engine efficiency is largely dependent on further increases in turbine inlet temperatures that will place even greater demands on blade cooling procedures.
In the numerical simulations, it is possible to impose temperature differences, heat flux levels, and even rotation effects that are difficult to establish in experimental programs. Thus, LES offers a way to gain fundamental information about turbine cooling flows to augment what can be learned from experiments. Such information can be used to refine turbulence models for use with the Reynolds-averaged equations.
Design criteria for gas turbine engines tend to result in maximum efficiency under takeoff conditions. However, at cruise conditions, Reynolds numbers are lower, and effects such as transition and bubble separation often result in a deterioration of turbine performance. Physics of such flows and the effects of passing wakes is not well understood and not accurately represented by current turbulence models. Here again, direct and LES can reveal aspects of the fundamental physics of such flows, providing details that are not easily measured experimentally. Such information can then guide the refinement of turbulence models for use with the Reynolds-averaged equations.
Work will continue on the ribbed channel simulations that address a flow characteristic of internal cooling channels in turbine blades. Effects of heat transfer and rotation will be considered. Work will continue on a step flow with heat transfer and channel flow with buoyancy effects. A new study to simulate flows in a circular pipe geometry that is characteristic of the coolant supply holes in film cooling applications is being initiated. Work will continue on using zonal embedded grids to minimize computer resources needed at high Reynolds numbers.
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